The present invention relates to fluorescent lamps and more particularly to a novel phosphor blend having two principal phosphors, with each phosphor emitting in a different region of the visible spectrum to increase the luminous efficacy of the fluorescent lamp.
It is known to utilize halophosphate phosphors in a fluorescent lamp for generating one of a plurality of standard "white" spectral power distributions. A typical 36 watt fluorescent lamp, having a particular spectral power distribution, e.g. the so-called "cool-white" color, utilizes the halophosphate phosphors to generate approximately 2850 lumens, yielding a luminous efficacy of about 79 lumens per watt (lm/W).
Luminous efficacies of greater than 80 lm/W. have not been generally available with practical fluorescent lamp phosphors, although increased efficacy is highly desirable in this age of energy scarcity and high cost.
It is well known to those skilled in the science of colorimetry that an infinite number of spectral power distributions (SPD) exist that have identical color coordinates (Hardy, "Handbook of Colorimetry", MIT Press (1936)). Any black body radiation of specified temperature has a known SPD and, therefore, a unique set of color coordinates. The locus of such sets for all temperatures is known as the black body locus. A color temperature may be computed for any other SPD lying near the black body locus. It is also well known that a theoretical luminous efficacy (TLE) for a phosphor of known SPD, responsive to absorption of ultraviolet power of a specified wavelength, can be calculated assuming a quantum efficiency (QE) of unity. Any real phosphor has a quantum efficiency less than unity and an experimental luminous efficacy (ELE), i.e. the product of TLE and QE, less than its TLE.
Another figure of merit is the color rendering index (CRI), which measures the degree to which the perceived colors of standard color plaques illuminated with a given SPD conform to those of the same plaques illuminated by a black body radiation with the same color temperature (Wyszecki and Stiles, Color Science, page 470 et seq., Wiley (1967)). Light from hot radiators such as a tungsten lamp or sunlight are characterized by a CRI close to 100. Deluxe fluorescent lamps have a CRI on the order of 80-90. Standard fluorescent lamps, used for most commercial and industrial lighting, typically have a CRI on the order of 50-70.